Barnett became well known after appearing on British breakfast station TV-am as a weekend presenter. She courted controversy in 1986 by breast-feeding her new daughter Bethany live on air.[citation needed]

Between 2002 and 2007, Barnett was host of UKTV Food's flagship food show, Great Food Live (previously known as Good Food Live) and its spinoffs Great Food Bites and Great Food Live Extra. In 2004/2005, Barnett also hosted the second series of the ITV1 cookery show, Too Many Cooks.

On 16 June 2007 Barnett presented her first radio programme on LBC 97.3, when she stood in for Chris Hawkins on Saturday afternoon. Barnett soon returned to the air filling in for more presenters, including Jim Davis' Lifestyle show. As part of a new Sunday schedule she was given a new 2-hour programme talking about food.

A further change on 7 January 2008 saw Barnett move to weekday afternoons, 1-4pm, as part of the new weekday line up. The programme is a topical debate featuring aspects of news and views, in which the public are encouraged to call in with their views.

Barnett continued to present the Sunday Food programme as well as the new weekday afternoon programme until 20 January 2008 when Bill Buckley replaced her on Sundays so she could concentrate on the weekday programme. Her weekday programme ended in September 2010.

Since January 2011, she has presented a Sunday morning show on BBC London 94.9, and regularly stands in for Vanessa Feltz on the weekday morning show from 9am till 12pm.

In a show broadcast on 7 January 2009,[4] Barnett's topics included the MMR vaccine. She and some callers expressed negative opinions of the vaccination and conventional medicine in general, and disagreed that recent disease outbreaks should be blamed on parents who choose not to vaccinate their children. The views reflected those expressed in the late 1990s when the media took up concerns about possible linkage of the vaccine to autism raised by Dr Andrew Wakefield, at the time overhyped and since shown to be wrong.[5] When a nurse phoned into the show and asked Barnett if she knew what was in the MMR vaccine, Barnett admitted that she did not.[6]

The discussion attracted criticism of its accuracy and possible negative influence on public health by doctor and journalist Ben Goldacre, who described the show as "irresponsible, ill-informed, and ignorant".[7]

Following Goldacre's criticisms, which he placed on his blog along with an audio clip of the show in question, the companies that aired Barnett's show (LBC and Global Radio) issued legal threats[7][8] to force removal of the audio clip on copyright grounds.[9] Goldacre removed the audio, but it, and transcripts prepared from it, have been made available by others via alternative sources[10][11] in what the blog Techdirt called a "Streisand effect", alluding to the wider publicity arising from an attempt to remove material from the internet.[12] Following this, the controversy received wider attention.[13][14]

After the broadcast, Barnett admitted on her blog that she "did not have the facts to hand...[was] ill informed...As a responsible broadcaster I should have been better prepared" when she discussed her claims with medical professionals who called in to question her statements.[15] Subsequently to this, all comments submitted to the blog entries dealing with this episode were removed from her site (although they have been archived elsewhere[16]). According to Barnett's agent, Robert Common, they contained "extremely personal and abusive comments"[17] although no such examples have been provided, and third-party archives of the comments do not appear to support this assessment.

This house ... expresses its disappointment that ill-informed comments by presenters such as Jeni Barnett on her LBC radio show will continue to cause unfounded anxieties for many parents and are likely to result in some parents choosing not to vaccinate their children ..."[18]

Barnett's broadcast was the recipient of complaints to OfCom, and Ofcom investigated the show. Ofcom's found that

At times, it appeared that during this broadcast the presenter relied upon her anecdotal experience and was not adequately briefed on the wider public health issues and prevailing medical advice which this debate would undoubtedly also touch upon. For example, at times the schedule of other childhood immunisations were confused with the MMR triple vaccine schedule, and no reference at all was made to any current research contesting a link between autism and the MMR vaccine yet several anecdotal references were made supporting a link.[19]

Based on the inclusion of alternate viewpoints from those expressed by Barnett, namely callers that included medical professionals criticising Barnett's statements, Ofcom's final ruling was that the broadcast did not violate rules against misleading portrayals of factual matters or undue prominence to minority views and opinions on matters of political or industrial controversy.[19]